The present invention relates to a levitation melting furnace which melts electrically conductive material by inductively heating the conductive material placed in an alternating magnetic field, the distribution thereof is adjusted to generate such electromagnetic force as to float the conductive material.
FIG. 4 is a cross sectional perspective view of a conventional levitation melting furnace. Referring now to FIG. 4, a cylindrical crucible 1 includes an electrically conductive metal segments 11, each having a cooling water conduit 4, and insulation layers 12 which insulate the adjoining segments 11 from each other. The segments and insulation layers are laminated one after the other to form the side and bottom of the cylindrical crucible 1. The molten metal can be tapped from a hole 5 of bottom of a crucible through the conduit 6. The hole diameter of the conduit 6 is larger than the diameter of the hole 5. An upper induction coil 2 is wound around the upper part of the crucible 1 for melting the metal in the crucible and for exerting electromagnetic force horizontally to a molten metal 8 to stabilize the floated molten metal 8. A lower induction coil 3 exerts levitational force strong enough to float the molten metal 8. The levitation melting furnace of FIG. 4, which pinches the diameter of the tapping hole 5 and expands the hole diameter of the tapping conduit 6 to exert the levitational force to the molten metal 8 by the lower induction coil 3, is disclosed in the Japanese Unexamined Patent Publication No. Hei. 7-249483.
FIG. 5 shows the external appearance of the conventional segment 11. Referring now to FIG. 5, the cooling water conduit 4 is inserted into the segment 11. Both ends of the cooling water conduit 4 are connected to respective cooling water feed pipes 9. The cooling water feed pipe 9 has a spigot joint 10 at an end thereof. As shown in FIG. 5, there remains no choice but to insert the induction coils upward from the bottom of the crucible due to the diameter differences between the crucible and induction coils. And, it is difficult to connect the cooling water feed pipes 9 beneath the segment 11 due to the space limitations. Therefore, the cooling water feed pipes 9 and the spigot joints 10 are aligned at first on the extensions of the segment 11. Then, the cooling water feed pipes 9 are bent outward and connected to the cooling water piping after the upper and lower induction coils 2 and 3 are mounted on the side of the crucible 1.
FIG. 6A is an induced current distribution on the crucible 1, FIG. 6B an induced current distribution on the molten metal 8, and FIG. 6C a distribution of force exerted to the molten metal 8, which are obtained by energizing the lower induction coil 3 of the levitation melting furnace which exerts levitational force to the molten metal by means of the narrowed diameter of the tapping hole 5 and the expanded diameter of the tapping conduit 6. In these figures, the symbols .smallcircle. and .quadrature. represent the induced currents flowing in opposite directions to each other in the crucible 1 and molten metal 8. The magnitudes of the current are indicated in proportion to the size of the symbols. The magnitudes of the force exerted to the molten metal 8 are indicated by the length of the arrows in FIG. 6C.
Since the currents flow in the opposite directions in the molten metal 8 and the crucible 1 surrounding the molten metal 8, repulsive electromagnetic force is exerted between the molten metal 8 and the crucible 1. When the repulsive electromagnetic force exceeds the gravitational force, the molten metal 8 is not tapped through the tapping hole 5. When the electromagnetic force around the tapping hole 5 is not so strong as to sustain the weight of the molten metal 8, the molten metal 8 is tapped through the tapping hole 5.
In the conventional levitation melting furnace which narrows the diameter of the tapping hole 8, the induced current is localized on the surface of the tapping hole, and the current induced in the molten metal 8 becomes larger toward the bottom of the molten metal 8. Therefore, strong force is exerted to the bottom of the molten metal 8. The levitational force exerted to the bottom of the molten metal 8 is 4 to 5 times as large as the levitational force generated by the lower induction coil 3, located around the tapping hole 5 having the diameter same with the hole diameter of the tapping conduit 6 and energized with the same current value. Therefore, sufficient levitational force is exerted to the molten metal 8 in spite of the reduced current feed to the lower induction coil 3.
The crucible 1 generates heat by the current induced therein by the upper and lower induction coils 2 and 3. The heat radiated from the molten metal to the crucible is carried away by the cooling water flowing through the cooling water conduits 4 in the segments 11.
In the above described conventional furnace, when the electric power fed to the upper induction coil and/or the lower induction coil is increased to improve the productivity by enlarging the furnace capacity or by shortening the melting period of time, the adjacent portion of the molten metal tapping hole is overheated, since the currents, induced in the crucible by the upper and lower induction coils and concentrating around the molten metal tapping hole, increase. Therefore, the maximum electric power fed to the upper induction coil and/or the lower induction coil is limited.
If the electric power fed to the lower induction coil is limited to suppress the overheating around the molten metal tapping hole, sufficient levitational force may not be obtained when the furnace capacity is enlarged.
For suppressing the overheating around the molten metal tapping hole by the conventional cooling scheme, it is necessary to expand the diameter of the cooling water conduit. Then, it becomes necessary to elongate the circumferential length of the segment to expand the diameter of the cooling water conduit without changing the relative spacing between the upper induction coil and the molten metal. However, since the elongated circumferential length of the segment reduces the levitational force exerted to the entire molten metal, it becomes difficult to lift the molten metal.
The method for preventing overheat by increasing the flow rate of the cooling water subjects to the other problems, since the pressure loss of the cooling water is proportional to the square of the flow rate of the cooling water and since the flow rate of the cooling water in these kinds of furnaces has been already from 13 to 15 m/sec which brings the pressure loss at the cooling water inlet of the furnace to limit thereof.
It takes time to replace the upper and lower induction coils, since it is necessary to heat and bend back the cooling water feed pipes prior to disengaging the induction coils.